Mainz surprised the champions as they ran out of 3-1 winners, but they should have won after an excellent performance by a more significant margin.
Early goals from Burkhardt and Barreiro sent them 2-0 ahead before Lewandowski pulled one back before the break.
Bayern started the second half well, with Lewandowski superbly denied by Hack in the six-yard box before the hosts caught Bayern out on the counter, and Barreiro’s deflected effort extended their lead.
They had numerous chances to extend their lead, with Onisiwo striking the bar and Burkhardt also going close, but ultimately they did enough to wrap up the victory over a sub-par Bayern.
MAINZ (3-5-2): Robin Zentner; Stefan Bell, Alexander Hack, Moussa Niakhate; Silvan Widmer, Anton Stach, Leandro Barreiro, Dominik Kohr, Aaron Caricol; Jonathan Burkhardt, Karim Onisiwo.
MAINZ SUBS: Finn Dahmen, Jeremiah St Juste, Jean-Paul Boetius, Marcus Ingvartsen, Daniel Brosinski, Kevin Stoger, Anderson Lucoqui, Merveille Papela, David Nemeth.
BAYERN (4-2-3-1): Sven Ulreich; Benjamin Pavard, Niklas Sule, Lucas Hernandez, Alphonso Davies; Leon Goretzka, Joshua Kimmich; Marcel Sabitzer, Eric-Maxim Choupo-Moting, Serge Gnabry, Robert Lewandowski.
BAYERN SUBS: Manuel Neuer; Omar Richards, Leroy Sane, Marc Roca, Tanguy Nianzou, Corentin Tolisso, Gabriel Vidovic, Jamal Musiala, Josip Stanisic.
GOOOAAALLL! MAINZ, TAKE THE LEAD! The hosts deserve their lead after a superb opening period of the game. It comes from a mistake by Ulreich at the back, his pass out is poor and is easily picked up by Kohr, who heads it to Burkardt, and he then fires the ball past the keeper and into the back of the net. It’s 1-0!
GOOOAAALLL! MAINZ DOUBLE THEIR LEAD! It comes from another set piece, as a free-kick is whipped into the box from the right, flicked on at the near post by Stach and then met at the back post by Niakhate, and he has a simple tap-in from close range. He couldn’t miss it, and he made it 2-0!
GOOOAAALLL! LEWANDOWSKI MAKES IT 2-1! The lovely move by Bayern on the edge of the penalty area, as Sabitzer flicks it to Choupo-Moting, who could go for a goal himself. However, he opts to pass to Lewandowski, and he turns and fires the ball into the bottom right corner of the net.
GOOOAAALLL! MAINZ, MAKE IT 3-1! The hosts quickly catch Bayern out at the back, with Stach flicking the ball to Widmer, who turns Davies and then lays it off to Barreiro on the edge of the box. The shot from Barreiro takes a big deflection off Pavard, but this sends it past Ulreich, onto the bar and into the back of the net.
So, what’s next?
The result doesn’t change the table for Bayern, but Mainz moves up to ninth and following up for them is a trip to Hertha Berlin. Meanwhile, Bayern faces Stuttgart at the Allianz Arena.
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